Referenza:
▪ Makovicky, E., Topa, D., Stoeger, B. (2018): The crystal structures of heptasartorite, Tl7Pb22As55S108, and enneasartorite, Tl6Pb32As70S140, two members of an anion-omission series of complex sulfosalts from Lengenbach, the Swiss Alps, and comparison with the structures of As–Sb sartorite homologues. European Journal of Mineralogy, 30, 1-16.

Abstract:
Crystal structures of heptasartorite and enneasartorite, two members of the sartorite anion-omission series were refined from the material from the Lengenbach deposit, Swiss Alps. Heptasartorite, Tl7Pb22(As54Sb)Σ55S108, is monoclinic, space group: P21/c (SG#14), a 29.2691(20), b 7.8768(5), c 20.1275(15) Å, β 102.065(2)°, unit-cell volume V = 4537.8(5) Å3. Enneasartorite, Tl6Pb32As70S140, is monoclinic, space group P21/c (SG#14), and lattice parameters a = 37.612(6), b = 7.8777(12), c = 20.071(3) Å, β = 101.930(2)°, and V = 5818.6(15) Å3. Both structures are typical structures of an N = 3 homologue of the sartorite homologous series with zig-zag walls of (Pb, Tl) trigonal coordination prisms separating (primarily) As-based slabs of structure configured to accommodate lone electron pairs of As. The sevenfold superstructure of heptasartorite is based on a sequence of crankshaft chains: on a surface of a fundamental double-ribbon in the As-rich structure slab one can recognize a W-shaped chain with five As and one mixed coordination polyhedron in which As and a large cation combine, a 4-member chain, a pair of As polyhedra, and two isolated As polyhedra. In the here redetermined structure of enneasartorite, one more 4-member chain of As polyhedra has been added, making the environment of the W-shaped chain symmetric. One of the isolated As polyhedra became a mixed-cation site. In both structures, the S site chelated by the As atoms of the W chain becomes vacant, generating the anion-omission derivatives of the ideal PbAs2S4 motif. The chain motifs differ from those in the Sb–As homologues of sartorite, which display only two-fold superstructures.

Marco E. Ciriotti

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